Measurements are presented for the annihilation rates of thermalized positrons in a variety of substances, including noble gases, simple inorganic molecules, hydrocarbons, substituted hydrocarbons, and aromatics. The measurements were performed using trapped clouds of room-temperature positrons, into which substances under test were introduced as low-pressure gases, so that only two-body interactions were involved. These data are compared with other values in the literature, and a compilation of annihilation rates is presented. The measuremetns illustrate the importance of both chemical composition and the vibrational modes of excitation of the molecules in determining the annihilation rates. The anomalously high annihilation rates observed for large molecules provide evidence for the existence of long-lived resonances. The nature of these resonances is not yet understood, and the data presented are expected to provide useful constraints for the development of theoretical models.

• Received 12 September 1994

DOI:https://doi.org/10.1103/PhysRevA.51.473